Dyestuff compositions are widely employed to color natural and synthetic fibers. Such compositions generally contain, in addition to a dyestuff component, a dispersant for the dyestuff. In the dyestuff composition, the dispersant generally serves three basic functions:
(1) it assists in reducing the dye particles to a fine size in a grinding operation;
(2) it maintains the dispersing medium for the dyestuff; and
(3) it serves as a diluent.
Sulfonated compounds are frequently employed as dye dispersants and may be obtained from two major sources, sulfonated lignins from the wood-pulping industry prepared from the lignin by-products of a sulfite or a kraft pulping process, and napthalene sulfonates derived as by-products from the petroleum industry.
In a kraft pulping process, lignin is obtained as a by-product from the spent pulping liquor, known as black liquor, where lignocellulosic materials, such as wood, straw, cornstalks, bagasse, and the like are processed to separate the cellulosic pulp from the lignin. In kraft pulping, the wood is subjected to the effects of strong alkali wherein the lignin forms a soluble sodium salt in the alkaline region which is separated from the cellulose and dissolves in the pulping liquor. The lignin is then recovered from the black pulping liquor by reducing the pH of the liquor to precipitate the lignin therefrom.
Lignin obtained from the kraft, soda, or other alkaline processes is not recovered as a sulfonated product, but is sulfonated, if desired, by reacting the material with a bisulfite or sulfite compound. Sulfonated lignins are understood to be lignins containing at least an effective amount of sulfonate groups to give water-solubility in moderately acid and higher pH solutions. The advantages of employing sulfonated lignins as dispersants in dyestuff compositions are due to their unique physical properties which include good capability to many dye systems, outstanding dispersant characteristics at ambient and elevated temperatures, and availability. There are, however, certain disadvantages in employing such lignins, whether sulfite process-derived lignins, or sulfonated kraft lignins, as dispersants. One negative factor in the use of lignins as dyestuff additives is that certain salts of the sulfonated lignins, particularly sodium and ammonium salts, tend to cause oxidative reduction and a color loss in dye systems using azo dyestuffs in the dyeing operation.
Color loss in the use of sulfonated lignins in azo dyestuff systems is caused by the action of the sulfonated lignins upon the --N.dbd.N-- azo groups in the dyestuff. Elimination of the nitrogen double bond by reduction to the hydrazo structure will cause loss of color, and if carried to complete rupture, will cause a destruction of the color body itself. In kraft lignins, functional groups capable of complex formation, such as catechol and phenol, can undergo numerous reactions in the presence or absence of oxygen which have the potential to reduce the azo dye color body.
The azo dye reduction caused by the use of lignosulfonate salts as dispersants in azo dye-containing systems is greatly increased when such azo dye systems contain soluble copper impurities. Soluble copper contaminants present in the dye system tend to catalyze the oxidative lignin reactions, thereby further aggravating azo dye reduction and color loss caused by the sulfonated lignins. Copper impurities may enter a dye system in the dyestuff itself, in additives applied to the system, or by contamination of the dye system from piping and handling vessels. Ionic copper by itself in the absence of lignin can undergo weak chelation with certain dyestuffs which results in shade distortions. While ethylenediaminetetraacetic acid (EDTA), a strong chelate compound, can alleviate azo dye reduction by metals, it is only moderately effective when employed in combination with sodium or ammonium lignosulfonate salt dispersants. This is due to the fact that the lignosulfonates are chelates themselves and therefore capture a portion of the metal salt impurities, that is, quantities of copper ion will remain in association with functional groups in the lignin molecule, such as the catechols and phenols.